1. Field of the Invention
The-present invention relates to optical disk reproducing methods and apparatus for reproducing an optical disk such as a CD-ROM which has stored a large amount of voice, character, image information and which has been connected to a personal computer, especially a lap-top computer.
2. Description of the Related Art
Recently, audio CDs as well as optical disks such as CD-ROMs have spread and efforts have been put into the development of optical disks of high performance in the related fields. Especially, since the optical disk is capable of recording/reproducing a large amount of data at high speed, a CD-ROM player as a data reproducing device has recently spread rapidly as the central existence of multimedia.
A conventional optical disk reproducing apparatus will be described next using a CD-ROM player as an example. FIG. 10 is a block diagram of the conventional CD-ROM player. Reference numeral 1 denotes a CD-ROM as an optical disk; 2, a spindle motor which rotates the CD-ROM 1; and 3, an optical pickup as optical reproducing means which reads information recorded on the CD-ROM 1.
The optical pickup 3 is comprised of a mechanical section and an optical section. The mechanical section is comprised of an object lens 4 which focuses light onto a recording surface of the CD-ROM 1, and an object lens actuator 5 integral with the object lens 4 to move the object lens 4 in a direction normal to a surface of the CD-ROM 1 (hereinafter referred to as "focusing direction") and in a radial direction of the CD-ROM 1 (hereinafter referred to as "tracking direction"). The optical section is comprised of a semiconductor laser, various prisms and an optical sensor 6.
A signal from the optical sensor 6 is delivered to a focus servo control unit 8, a tracking servo control unit 9, and an information signal detector 11 which produce a focus error signal, a tracking error signal and an information detection signal to provide focus control and tracking control of the object lens 4 and signal processing by a signal processor 12, respectively.
A reference numeral 7 denotes a linear motor which is controlled by an access control unit 10 when the optical pickup 3 moves greatly across tracks to thereby move the optical pickup 3 to a target track.
Reference numeral 13 denotes an interface control unit which outputs an information detection signal obtained as the result of information processing by a controller 15 through the interface control unit 13 and a PCMCIA card 22 to a host device (such as a computer) (not shown). Of the signals detected by the information signal detector 11, voice information is produced as a voice signal by the signal processor 12, and outputted by the controller 15 through the interface control unit 13 to the host device (computer). Alternatively, it is delivered through headphone terminals 23 to an external voice output device which then produces a corresponding voice output.
The information detection signal is delivered to the spindle motor control unit 14 to rotate the spindle motor 2 at constant rotational speeds. The controller 15 controls the whole operation of the CD-ROM player.
FIG. 11 is a perspective view of a CD-ROM player 21 as a conventional optical disk reproducing apparatus of an external connection type (hereinafter referred to as a portable type) through the interface (PCMCAia) card 22 to a lap-top personal computer (not shown). Reference numeral 23 denotes headphone terminals to output reproduced voice information from the CD-ROM 1 to the outside. By connecting a headphone (not shown) to the headphone terminals 23, voice is reproducible.
Servo control of the conventional CD-ROM player, thus constructed, will be described next. The CD-ROM has an information track provided in a spiral form in which an information signal is recorded. Since the track interval is very small (1.6 .mu.m), a light spot is required to be positioned with high accuracy relative to the rotating CD-ROM 1 to detect an information signal accurately from the track.
A detailed schematic of the conventional focus servo control unit 8 is shown in FIG. 12, in which reference numeral 15 denotes the controller of FIG. 10; 16, a phase compensation filter to stabilize the servo system; 17, a servo amplifier; 18, a servo gain setting circuit; and 19, a driver.
The operation of the conventional focus servo control unit 8, thus constructed, will be described next. A focus error signal obtained from the optical pickup 3 is inputted to the phase compensation filter 16 to stabilize the servo system. It is then amplified by the servo amplifier 17 and delivered through the driver 19 to the object lens actuator 5 for driving purposes.
The servo gain setting circuit 18 is provided to maintain the servo gain at a fixed value in spite of variations in the optical signal from the optical pickup 3 and the sensitivity of the object lens actuator 5. The amplification factor of the servo amplifier 17 is set by the controller 15 so that a desired drive output is obtained from the driver 19.
Tracking servo control will be described next. FIG. 13 shows an open loop characteristic of the tracking servo. For example, the standard value of the maximum eccentric quantity of the CD-ROM 1 is .+-.70 .mu.m whereas the actual mechanical eccentric value is about .+-.120 .mu.m when the eccentricity of the motor shaft and a turn table are considered.
In order to avoid a reduction of an error rate of the reproduced signal, a deviation of a track (optical pit) is required to be suppressed to within .+-.0.1 .mu.m.
Thus, a servo gain in a low frequency (less than 30 Hz) region is set at EQU 20 log (120/0.1)=61.6 dB.
While the maximum eccentric acceleration is 0.4 G according to the above-mentioned standards in a high-frequency region servo gain, the gain intersection is set at about 1 kHz to improve resistance to vibrations of the optical disk reproducing apparatus and to suppress a high-harmonic resonance frequency.
As described above with reference to the standard values of an optical disk such as a CD-ROM, its roundness and flatness involve errors, which cooperate with an error involved in the optical disk reproducing apparatus itself to rotate the optical disk while vibrating same a little in a radial direction (hereinafter referred to as "eccentricity") or rotating same while vibrating same vertically (hereinafter referred to as "plane vibrations"). A quantity of plane vibrations/eccentricity varies from optical disk to optical disk. Some commercially available optical disks exceed the standards of plane vibrations and eccentricity.
When information to be reproduced is voice information in the conventional portable optical disk reproducing apparatus, the voice output is delivered to the host or to external speaker means through the headphone terminal of the apparatus.
In order to regard the portability of the portable optical disk reproducing apparatus as important, an optical disk reproducing apparatus in which the voice information outputting speaker means is built in the optical disk reproducing device has been developed. The influence of the optical reproducing means on the servo control varies between when the portable type optical disk reproducing apparatus is usually used to reproduce character/image information (inclusive of outputting voice information through the headphone terminals, which applies hereinafter to a similar case) and when the voice information is output by the built-in speaker means.
When the voice information is outputted through the built-in speaker means, the speaker means is provided adjacent the optical reproducing means (optical pickup) due to the problem of the size and spacial arrangement of the reproducing apparatus itself, and, hence, the influence on the voice output due to vibrations of the speaker means occurs. Thus, when the set value of the servo gain is designed in conformity to the standards, the reproducing apparatus operates at the fixed set value in spite of influence due to vibrations of the built-in speaker means. Thus, when information in the optical disk is reproduced, the focus and tracking errors will exceed the predetermined allowable values to thereby bring about no normal reproduction.
For example, FIG. 14 shows a state where the focus error has exceeded an allowable range. As shown, when the optical disk information is reproduced when the quantity of plane vibrations exceeds the standard, the focus error signal will not fall within the allowable range. Thus, in order to reproduce. information in the optical disk normally in such a case, the focus servo gain is required to be increased or otherwise the characteristic of the servo loop gain (or phase compensation filter) is required to be changed.
In order to increase the servo gain, the amplification factor of the servo amplifier could be increased. However, in this method, the focus error signal is suppressed by increasing the servo gain. Since the gains in both the servo and higher frequency regions increase simultaneously, the reproduced output reacts sensitively to flaws on, and/or noise in, the optical disk, and the playability of reproduced information over the flaws on the optical disk in the normal disk reproduction (involving no voice output by the built-in speaker means) would be deteriorated. In addition, excessive control is provided on an optical disk whose quantity of plane vibrations is small, so that power consumption would increase.
Alternatively, an increase in the low-frequency region gain alone based on a change of the phase compensation filter characteristic could be considered. Since the high-frequency region gain according to this method is substantially equal to that provided by the conventional one, the playability is not deteriorated. However, if the low-frequency region gain of the servo system is increased, the stability of the servo system is impaired accordingly, so that this method is not so recommendable. Thus, the problems of excessive control and power consumption in the regular optical disk reproduction are not solved.
As described above, in the conventional optical disk device, the servo gain is preferably set low for flaws on the optical disk whereas the servo gain is preferably set high for external disturbances such as sounds and/or vibrations. Thus, since there is no method of satisfying those contradictory requirements, the designer must compromise with any one of those requirements.
The present invention intends to solve the above problem. It is an object of the present invention to provide an optical disk reproducing method and apparatus which is capable of reproducing voice information normally with the speaker means built in the optical disk apparatus even when errors exceeding the standards occur and reproducing information from a CD-ROM even in a regular manner (state of use) with low power consumption in a stabilized manner.